SQL Injection (SQLi)

SQL Injection (SQLi) refers to an injection attack wherein an attacker can execute malicious SQL statements (also commonly referred to as a malicious payload) that control a web application’s database server (also commonly referred to as a Relational Database Management System – RDBMS). Since an SQL Injection vulnerability could possibly affect any website or web application that makes use of an SQL-based database, the vulnerability is one of the oldest, most prevalent and most dangerous of web application vulnerabilities.

By leveraging an SQL Injection vulnerability, given the right circumstances, an attacker can use it to bypass a web application’s authentication and authorization mechanisms and retrieve the contents of an entire database. SQL Injection can also be used to add, modify and delete records in a database, affecting data integrity.

How SQL Injection works

In order to run malicious SQL queries against a database server, an attacker must first find an input within the web application that is included inside of an SQL query.

In order for an SQL Injection attack to take place, the vulnerable website needs to directly include user input within an SQL statement. An attacker can then insert a payload that will be included as part of the SQL query and run against the database server.

The following server-side pseudo-code is used to authenticate users to the web application.

Once the query executes, the result is returned to the application to be processed, resulting in an authentication bypass. In the event of authentication bypass being possible, the application will most likely log the attacker in with the first account from the query result — the first account in a database is usually of an administrative user.

What’s the worst an attacker can do with SQL?

SQL is a programming language designed for managing data stored in an RDBMS, therefore SQL can be used to access, modify and delete data. Furthermore, in specific cases, an RDBMS could also run commands on the operating system from an SQL statement.

Keeping the above in mind, when considering the following, it’s easier to understand how lucrative a successful SQL Injection attack can be for an attacker.

An attacker can use SQL Injection to bypass authentication or even impersonate specific users.

One of SQL’s primary functions is to select data based on a query and output the result of that query. An SQL Injection vulnerability could allow the complete disclosure of data residing on a database server.

Since web applications use SQL to alter data within a database, an attacker could use SQL Injection to alter data stored in a database. Altering data affects data integrity and could cause repudiation issues, for instance, issues such as voiding transactions, altering balances and other records.

SQL is used to delete records from a database. An attacker could use an SQL Injection vulnerability to delete data from a database. Even if an appropriate backup strategy is employed, deletion of data could affect an application’s availability until the database is restored.

Some database servers are configured (intentional or otherwise) to allow arbitrary execution of operating system commands on the database server. Given the right conditions, an attacker could use SQL Injection as the initial vector in an attack of an internal network that sits behind a firewall.

The anatomy of an SQL Injection attack

An SQL Injection needs just two conditions to exist – a relational database that uses SQL, and a user controllable input which is directly used in an SQL query.

In the example below, it shall be assumed that the attacker’s goal is to exfiltrate data from a database by exploiting an SQL Injection vulnerability present in a web application.

Supplying an SQL statement with improper input, for example providing a string when the SQL query is expecting an integer, or purposely inserting a syntax error in an SQL statement cause the database server to throw an error.

Errors are very useful to developers during development, but if enabled on a live site, they can reveal a lot of information to an attacker. SQL errors tend to be descriptive to the point where it is possible for an attacker to obtain information about the structure of the database, and in some cases, even to enumerate an entire database just through extracting information from error messages – this technique is referred to as error-based SQL Injection. To such an extent, database errors should be disabled on a live site, or logged to a file with restricted access instead.

Another common technique for exfiltrating data is to leverage the UNION SQL operator, allowing an attacker to combine the results of two or more SELECT statements into a single result. This forces the application to return data within the HTTP response – this technique is referred to as union-based SQL Injection.

The following is an example of such a technique. This can be seen on testphp.vulnweb.com, an intentionally vulnerable website hosted by Acunetix.

The following HTTP request is a normal request that a legitimate user would send.

Although the above request looks normal, the artist parameter in the GET request’s query string is vulnerable to SQL Injection.

The SQL Injection payload below modifies the query to look for an inexistent record by setting the value in the URL’s query string to -1 (it could be any other value that does not exist in the database, however, an ID in a database is less likely to be a negative number).

In SQL Injection, the UNION operator is commonly used to allow an attacker to join a malicious SQL query to the original query intended to be run by the web application. The result of the injected query will be joined to the result of the original query, allowing an attacker to exfiltrate data out of a database by obtaining values of columns from other tables.

The above example proves that the query to the database can be modified to return data which an attacker may want to extract. The following example shows how an SQL Injection payload could be used to exfiltrate data from this intentionally vulnerable site.